At Tokyo Electric Power's Fukushima Daiichi plant, three reactors shut down automatically as designed after a magnitude 8.9 earthquake struck off the northeast of Japan on Friday afternoon. The quake also caused diesel-powered generators used to cool the reactors to stop operating, leaving the utility company with a shortage of coolant to bring the reactors to a safe temperature.

Japan is battling to stave off a nuclear disaster after an explosion at a north-eastern nuclear plant in the wake of the enormous earthquake and tsunami.

Authorities are evacuating tens of thousands of residents living within a 12 mile (20km) radius of the Fukushima Daiichi plant and those within 6 miles of a second installation in Futuba, 150 miles north of Tokyo.